The present invention relates to an offshore production platform and methods of installing such a platform on a production site.
Jackup-type offshore production platforms, such as oil platforms for example, generally comprise legs, usually three in number, bearing on the seabed, and a deck mounted so as to be able to move with an adjustable height on the legs and carrying, in particular, production equipment and living premises.
The bottom end of each leg is fitted with a foot or stand designed to bear on the seabed when the platform is installed on a production site. This foot or stand of each leg provides a limited area of contact with the seabed, allowing the self-weight of the platform to be supported only under calm sea conditions and for a relatively short time.
For this reason, the legs of the platform have to be solidly anchored in the seabed for a long stay corresponding to a normal period of a production campaign.
A known method of achieving this is to fix each foot of the legs of the platform using piles driven deeply into the ground. But, given the size of such piles, this measure is a lengthy and costly operation because it requires intervention of heavy equipment whose operating cost is extremely high.
Another solution commonly adopted for anchoring the legs of the platform in the seabed involves combining a caisson, open at the bottom and fitted with a valve, with each leg. Each caisson is thrust into the seabed under the weight of the whole platform. In other cases, each caisson is combined with a pumping unit to reduce the caisson internal pressure and to cause this caisson to penetrate into the seabed to the required depth. These caissons combined with a pumping unit are referred to by specialists under the name of “suction pile”.
Each caisson is generally stationary and extends below the foot or the stand of the corresponding leg; this has drawbacks.
During transportation of the preassembled platform from the erection site to the production site, either by floating the platform or by barge, these caissons, which are several meters high, are in the water and create high resistance to platform forward movement and thus additional energy expenditure.
Furthermore, this arrangement creates problems, when mounting the caissons onto the bottom end of the legs of the platform. The bottom end of each leg is submerged in most cases, so much so that handling these caissons requires heavy, complex infrastructure, due to the volumes to be controlled, because these caissons have a diameter of around 17 to 20 meters and a height that can attain several meters.